Though finally solved, the mystery of the creek that was a leak is an example of how utility districts in the US can't account for 6 billion gallons of drinking water each day. If all that lost water were collected over the course of a year, it would fill Gatun Lake, the huge reservoir that feeds the Panama Canal.

Georgia recently began requiring counties seeking water-withdrawal permits to first check their waterworks for leaks. Three other states, including Tennessee, are tightening water audit requirements, and the American Water Works Association (AWWA) has persuaded 300 communities to take part in a public-service campaign called "Only Tap Water Delivers," in part prompted by mounting water losses.

Communities here in the South, one of the most water-rich corners of the US, are starting to put "leak detectives" on missing water cases.

According to the AWWA in Denver, the nation needs to spend some $250 billion in the next 20 to 30 years to upgrade its tap-water delivery systems, which were built primarily in three phases – the turn of the 20th century, the roaring '20s, and right after World War II. Most of that cost is likely to show up on consumers' water bills.

"We're at a turning point when it comes to our water infrastructure," says Greg Kail, AWWA spokesman. "Our generation has not experienced the cost of putting those pipes in the ground, and the end result is that water is going to be more expensive in the future. Water utilities throughout North American are ... taking steps [now] to try to mitigate that rate shock."

Historically, Americans haven't bothered too much with leaky pipes, confident that rivers and lakes will fill and tap water will remain cheap.

"If you're in a situation for many years where water is plentiful, there's not a lot of incentive to maintain reservoirs and pipe," says Peter Lavigne, an environmental studies professor at Western State College of Colorado in Gunnison. "But then you get a drought and you find major leaks, and all of a sudden it becomes critical."

Perhaps 5 percent of unaccounted-for water can be blamed on slow water meters, fire-hydrant testing, and firefighting. The vast majority of water loss – sometimes more than half of a district's total flow – is tied to undetected breaks that can trickle or gush for years. Revenue losses from that missing water, in turn, can hamper districts' ability to patch pipes.

The problem can be daunting, says Bill Thompson, general manager for the rural White House Utility District in White House, Tenn., which distributes drinking water across 640 square miles. A class action lawsuit against the manufacturers of polybutylene pipe, an inexpensive option popular in the 1980s and '90s, yielded a $950 million settlement, though manufacturers never admitted fault. Expected to last 30 years, the pipes, Mr. Thompson says, turn brittle and fail sometimes after only 10 years. What's more, federal and state dollars usually go toward making sure water is potable, not to maintaining infrastructure or leak detection.

"If you're going to tell all these municipalities to fix their leaks, you've got to provide the cash to do that," says Janet Ward of Atlanta's Department of Watershed Management. "A lot of Southern states are cutting taxes willy-nilly, so there's no revenues to put into this."

In times of shortage, water districts that opt to impose water-use restrictions may find it embarrassing to admit that they are losing millions of gallons. Drought-stricken Atlanta, for instance, loses at least 12 percent of its water every day. In Nashville, officials estimate that 677 million gallons, of a total 2.67 billion, disappear each month. Even in the midst of watering restrictions, Mount Pleasant, Tenn., still can't account for about half its water.

"Between 60 to 70 percent of leaks are not only hidden, but the utility doesn't know anything about them," says Bud Reed, the Gallatin, Tenn.-based sales manager for Flow Metrix, a supplier of acoustic leak-detection equipment.

Of course, no water system is perfectly tight. The closest may be Key West, Fla., where limited supplies of fresh water mean leak detection is a top priority. The fact is, only 1 percent of utility districts actually check for hidden leaks.

That's changed here in Morrow, where a business-minded chairman of the water utility board started asking questions six years ago about the water that went missing every day. A small crew was assembled, a van was bought, and a crack water-detective agency was born.

Inspecting pipe 500 feet at a time, the detectives locate the high-pitched whine of underground leaks using a kind of pipe sonar, computer software, and headphones. Now that Georgia is demanding more accountability from water districts, other counties are hiring the crew as consultants to crack their missing-water cases.

In six years, the crew found 352 leaks across 1,350 miles of pipe, saving Clayton County $12 million worth of water. Even as the county population spurted over the past six years, actual annual water production here dropped by nearly 1 billion gallons.